Photophysics and dynamics of the lowest excited singlet state in long substituted polyenes with implications to the very long-chain limit

Abstract

In this paper we explore the intramolecular relaxation processes within two long carotenoids, namely decapreno‐β‐carotene (M15) and dodecapreno‐β‐carotene (M19) with 15 and 19 conjugated double bonds (N), respectively. Amplified 200 fs pulses at 590 nm were used to excite the optically allowed S₀→S₂ (1 ¹A_g→1 ¹B_u) transition of the two carotenoids. The excited state dynamics were probed by continuum light between 400–890 nm in solvents with different polarizabilities. The transient absorption spectra consist of a bleaching region, due to loss of ground state absorption, and of an excited state absorption region at longer wavelengths, due to the S₁→S_n transition. The S_n state was assigned to an n ¹B_u state. The overall wavelength dependence of the measured kinetics could be well described by introducing three decay time constants. One reflects the S₁ lifetime (τ₁) and was determined to 1.1 and 0.5 ps for M15 and M19, respectively. A second lifetime, between 5 and 15 ps, was attributed to vibrational cooling in the ground state. A third decay time was in the subpicosecond range, and was ascribed to the vibrational redistribution and relaxation of the S₁ potential surface after being populated by the subpicosecond S₂–S₁ internal conversion. No significant change of the decay constants was observed for M15 embedded in a 77 K matrix. This shows that the relaxation rates are only influenced by intramolecular processes. The S₂ lifetime was shorter than the pulse duration and was estimated to be in the order of 100 fs. The S₀→S₂ transition of M15 in the liquid phase exhibits a 0.39 anisotropy at short times, while the S₁→S_n transition has an initial value of only 0.31. This corresponds to an angle of 23° between the transition dipoles. The measured S₁ rate constants were analyzed, together with decay constants of shorter carotenes, in terms of the energy gap law. When going from the shortest (N=5) to the longest (N=19) polyene, τ₁ decreases about 6000 times, i.e., from 3 ns to 0.5 ps. By using an empirical form of the energy gap law the 0–0 transition of S₁(2 ¹A_g)→S₀ was estimated to be located at 11 300 and 10 200±1 000 cm⁻¹ for M15 and M19, respectively. By fitting the excitation energies of all carotenes in the series (3≤N≤19) with a truncated two or three term expansion of a power series in 1/N the long‐chain limit values were extrapolated to be 11 000 and 3 500 cm⁻¹ for the 1 ¹B_u and 2 ¹A_g state, respectively. The implication of these limit values on the electronic structure of polyacetylene are discussed.